In Austrian patent No. 298,830 there has been disclosed a system of this character, used for example in the testing of inflated automobile tires by a holographic technique, wherein a source of coherent light (e.g. a laser) generates a pair of beams, namely an illuminating beam and a reference beam. The illuminating beam is trained upon the test object from which rays of that beam are reflected onto a receiving surface such as the photocathode of a television camera. The reference beam is directed onto the same receiving surface so as to produce thereon an interference pattern due to phase differences between the rays of that beam and those reflected by the test object, the latter rays being focused upon the receiving surface by a main objective. Two ancillary objectives, traversed by the illuminating beam and the reference beam, cause their rays to diverge on their way to the test object and to the receiving surface, respectively, by focusing them upon nearby points; the diverging reference beam is then mirrored into the path of the reflected light rays between the main objective and the receiving surface. The projected image of the test object, distorted by the interference pattern, is temporarily stored for comparison with a second such image taken at a later time in order to reveal intervening changes in the position or in the shape of the test object.
The described system can also be used to produce a holographic record of the test object, yet such a record does not lend itself to evaluation by visual inspection or simple electronic means.